Systolic [Ca2+ ]i regulates diastolic levels in rat ventricular myocytes.

KEY POINTS: For the heart to function as a pump, intracellular calcium concentration ([Ca2+ ]i ) must increase during systole to activate contraction and then fall, during diastole, to allow the myofilaments to relax and the heart to refill with blood. The present study investigates the control of diastolic [Ca2+ ]i in rat ventricular myocytes. We show that diastolic [Ca2+ ]i is increased by manoeuvres that decrease sarcoplasmic reticulum function. This is accompanied by a decrease of systolic [Ca2+ ]i such that the time-averaged [Ca2+ ]i remains constant. We report that diastolic [Ca2+ ]i is controlled by the balance between Ca2+ entry and Ca2+ efflux during systole. The results of the present study identify a novel mechanism by which changes of the amplitude of the systolic Ca transient control diastolic [Ca2+ ]i . ABSTRACT: The intracellular Ca concentration ([Ca2+ ]i ) must be sufficently low in diastole so that the ventricle is relaxed and can refill with blood. Interference with this will impair relaxation. The factors responsible for regulation of diastolic [Ca2+ ]i , in particular the relative roles of the sarcoplasmic reticulum (SR) and surface membrane, are unclear. We investigated the effects on diastolic [Ca2+ ]i that result from the changes of Ca cycling known to occur in heart failure. Experiments were performed using Fluo-3 in voltage clamped rat ventricular myocytes. Increasing stimulation frequency increased diastolic [Ca2+ ]i . This increase of [Ca2+ ]i was larger when SR function was impaired either by making the ryanodine receptor leaky (with caffeine or ryanodine) or by decreasing sarco/endoplasmic reticulum Ca-ATPase activity with thapsigargin. The increase of diastolic [Ca2+ ]i produced by interfering with the SR was accompanied by a decrease of the amplitude of the systolic Ca transient, such that there was no change of time-averaged [Ca2+ ]i . Time-averaged [Ca2+ ]i was increased by ß-adrenergic stimulation with isoprenaline and increased in a saturating manner with increased stimulation frequency; average [Ca2+ ]i was a linear function of Ca entry per unit time. Diastolic and time-averaged [Ca2+ ]i were decreased by decreasing the L-type Ca current (with 50 µm cadmium chloride). We conclude that diastolic [Ca2+ ]i is controlled by the balance between Ca entry and efflux during systole. Furthermore, manoeuvres that decrease the amplitude of the Ca transient (without decreasing Ca influx) will therefore increase diastolic [Ca2+ ]i . This identifies a novel mechanism by which changes of the amplitude of the systolic Ca transient control diastolic [Ca2+ ]i .

In the RyR2(R4496C) mouse model of CPVT, ß-adrenergic stimulation induces Ca waves by increasing SR Ca content and not by decreasing the threshold for Ca waves.

RATIONALE: mutations of the ryanodine receptor (RyR) cause catecholaminergic polymorphic ventricular tachycardia (CPVT). These mutations predispose to the generation of Ca waves and delayed afterdepolarizations during adrenergic stimulation. Ca waves occur when either sarcoplasmic reticulum (SR) Ca content is elevated above a threshold or the threshold is decreased. Which of these occurs in cardiac myocytes expressing CPVT mutations is unknown. OBJECTIVE: we tested whether the threshold SR Ca content is different between control and CPVT and how it relates to SR Ca content during ß-adrenergic stimulation. METHODS AND RESULTS: ventricular myocytes from the RyR2 R4496C(+/-) mouse model of CPVT and wild-type (WT) controls were voltage-clamped; diastolic SR Ca content was measured and compared with the Ca wave threshold. The results showed the following. (1) In 1 mmol/L [Ca(2+)](o), ß-adrenergic stimulation with isoproterenol (1µmol/L) caused Ca waves only in R4496C. (2) SR Ca content and Ca wave threshold in R4496C were lower than those in WT. (3) ß-Adrenergic stimulation increased SR Ca content by a similar amount in both R4496C and WT. (4) ß-Adrenergic stimulation increased the threshold for Ca waves. (5) During ß-adrenergic stimulation in R4496C, but not WT, the increase of SR Ca was sufficient to reach threshold and produce Ca waves. CONCLUSIONS: in the R4496C CPVT model, the RyR is leaky, and this lowers both SR Ca content and the threshold for waves. ß-Adrenergic stimulation produces Ca waves by increasing SR Ca content and not by lowering threshold.

Distinct circadian mechanisms govern cardiac rhythms and susceptibility to arrhythmia.

Electrical activity in the heart exhibits 24-hour rhythmicity, and potentially fatal arrhythmias are more likely to occur at specific times of day. Here, we demonstrate that circadian clocks within the brain and heart set daily rhythms in sinoatrial (SA) and atrioventricular (AV) node activity, and impose a time-of-day dependent susceptibility to ventricular arrhythmia. Critically, the balance of circadian inputs from the autonomic nervous system and cardiomyocyte clock to the SA and AV nodes differ, and this renders the cardiac conduction system sensitive to decoupling during abrupt shifts in behavioural routine and sleep-wake timing. Our findings reveal a functional segregation of circadian control across the heart's conduction system and inherent susceptibility to arrhythmia.

Increasing ryanodine receptor open probability alone does not produce arrhythmogenic calcium waves: threshold sarcoplasmic reticulum calcium content is required.

Diastolic waves of Ca(2+) release have been shown to activate delayed afterdepolarizations as well as some cardiac arrhythmias. The aim of this study was to investigate whether increasing ryanodine receptor open probability alone or in the presence of beta-adrenergic stimulation produces diastolic Ca release from the sarcoplasmic reticulum (SR). When voltage-clamped rat ventricular myocytes were exposed to caffeine (0.5 to 1.0 mmol), diastolic Ca(2+) release was seen to accompany the first few stimuli but was never observed in the steady state. We attribute the initial phase of diastolic Ca(2+) release to a decrease in the threshold SR Ca(2+) content required to activate Ca(2+) waves and its subsequent disappearance to a decrease of SR content below this threshold. Application of isoproterenol (1 micromol/L) increased the amplitude of the systolic Ca(2+) transient and also the SR Ca(2+) content but did not usually produce diastolic Ca(2+) release. Subsequent addition of caffeine, however, resulted in diastolic Ca(2+) release. We estimated the time course of recovery of SR Ca(2+) content following recovery from emptying with a high (10 mmol/L) concentration of caffeine. Diastolic Ca(2+) release recommenced only when SR content had increased back to its final level. We conclude that increasing ryanodine receptor open probability alone does not produce arrhythmogenic diastolic Ca(2+) release because of the accompanying decrease of SR Ca(2+) content. beta-Adrenergic stimulation increases SR content and thereby allows the increased ryanodine receptor open probability to produce diastolic Ca(2+) release. The implications of these results for arrhythmias associated with abnormal ryanodine receptors are discussed.

The sarcoplasmic reticulum and arrhythmogenic calcium release.

There is much evidence showing that some lethal ventricular arrhythmias arise from waves of Ca(2+) release from the sarcoplasmic reticulum (SR) that propagate along cardiac cells. The purpose of this review is to discuss the mechanism of production of these waves and how they depend on the properties of the SR Ca(2+) release channel or ryanodine receptor (RyR). The best-known method of producing Ca(2+) waves is by increasing the Ca(2+) content of the cell by either increasing Ca(2+) influx or decreasing efflux. Once SR Ca(2+) content reaches a threshold level a Ca(2+) wave is produced. Altering the properties of the RyR affects the threshold level of Ca(2+) required to produce a wave. Patients with a mutation in the RyR suffer from catecholaminergic polymorphic ventricular tachycardia, and this may be due to a decrease in the SR Ca(2+) threshold for wave production. Heart failure has also been suggested to result in Ca(2+) waves due to a leak of Ca(2+) through the RyR. We review the finding that these changes in RyR function will only result in Ca(2+) waves in the steady state if some other mechanism maintains the SR Ca(2+) content. The review concludes with a description of potential mechanisms for treating arrhythmias produced by Ca(2+) waves.

Flecainide and antiarrhythmic effects in a mouse model of catecholaminergic polymorphic ventricular tachycardia.

Recent studies have shown that flecainide may be an effective therapy to prevent life-threatening arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia. Several hypotheses have been advanced to explain the antiarrhythmic mechanism of flecainide, including Na(+) channel blockade and a direct inhibitory action on the ryanodine receptor. In this article, we review the current literature on the topic and summarize the elements of the existing debate.